Transcript chapter29_Sections 1-5.ppt

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 29
Neural Control
(Sections 29.1 - 29.5)
Albia Dugger • Miami Dade College
29.1 In Pursuit of Ecstasy
• Ecstasy (MDMA) makes some neurons in the brain release
an excess of a signaling molecule called serotonin
• Multiple doses of MDMA alter the structure of neurons that
secrete serotonin and decrease their number
• Sometimes these disruptions cause unwanted effects or even
death – a bad reaction to Ecstasy or any other drug requires
immediate medical help
Effects of Ecstasy
29.2 Evolution of Nervous Systems
• Signals move quickly through most animal bodies along an
information highway consisting of neurons; neuroglial cells
provide functional and structural support to neurons
• neuron
• An excitable cell that transmits electrical signals along its
plasma membrane, and chemical messages to other cells
• neuroglial cell
• Cell that supports neurons
The Cnidarian Nerve Net
• Cnidarians such as hydras and jellyfish have a nerve net
• Information flows in any direction among cells of a nerve net,
and there is no centralized, controlling organ that functions
like a brain
• nerve net
• Of cnidarians, a mesh of interacting neurons with no
central control organ
Hydra, a Cnidarian
• A nerve net controls
contractile cells in the
epithelium
Hydra, a Cnidarian
A nerve net
(highlighted
in purple)
controls the
contractile
cells in the
epithelium.
A Hydra, a cnidarian
Fig. 29.2a, p. 468
Bilateral, Cephalized Invertebrates
• Animals with a bilateral, cephalized body plan have three
types of neurons:
• Sensory neurons detect stimuli and send signals to
motor neurons or interneurons
• Interneurons integrate information and send signals to
one another or to motor neurons
• Motor neurons carry signals to effectors (muscles and
glands)
Key Terms
• sensory neuron
• Neuron that responds to a specific internal or external
stimulus and signals another neuron
• interneuron
• Neuron that receives signals from and sends signals to
other neurons
• motor neuron
• Neuron that receives signals from another neuron and
sends signals to a muscle or gland
Bilateral, Cephalized Invertebrates
• Simple bilateral animals such as planarians have a nervous
system with a cluster of ganglia at the head end, ventral
nerve cords running the length of the body, and nerves
extending out from the nerve cords
• Annelids (such as earthworms) and arthropods have paired
nerve cords that run along the ventral (lower) surface and
connect to a simple brain
Key Terms
• ganglion
• Cluster of nerve cell bodies
• nerve cord
• Bundle of nerve fibers running the length of a body
• nerve
• Neuron fibers bundled inside a sheath of connective tissue
Planarian
• Planarians are bilateral
animals with a simple
nervous system
• A pair of ganglia in the
head serves as an
integrating center
Planarian
clusters of
ganglia
pair of nerve
cords crossconnected
by lateral
nerves
B Planarian, a flatworm
Fig. 29.2b, p. 468
Earthworm
• In annelids (and
arthropods), a pair of
ganglia control the
muscles in each
segment
Earthworm
rudimentary
brain
ventral
nerve cord
ganglion
C Earthworm, an annelid
Fig. 29.2c, p. 468
Arthropods
Arthropods
brain
brain
optic lobe
(one pair, for
visual stimuli)
branching
nerves
paired ventral
nerve cords
ganglion
D Crayfish, a crustacean
(a type of arthropod)
E Grasshopper, an insect
(a type of arthropod)
Fig. 29.2d,e, p. 468
The Vertebrate Nervous System
• In vertebrates, the dorsal nerve cord evolved into a brain and
spinal cord, which make up the central nervous system
• Paired nerves of the peripheral nervous system connect the
brain and spinal cord to the rest of the body
Key Terms
• central nervous system
• Brain and spinal cord
• Most interneurons reside in the central nervous system
• peripheral nervous system
• Nerves that extend through the body and carry signals to
and from the central nervous system
Peripheral Nervous System
• Nerves of the peripheral system fall into two functional
categories: autonomic and somatic
• Autonomic nerves monitor and regulate the internal state of
the body; they control smooth muscle, cardiac muscle, and
glands
• Somatic nerves monitor the body’s position and the external
environment; they control skeletal muscle
Divisions of Vertebrate
Nervous Systems
Divisions of Vertebrate Nervous Systems
Central Nervous System
Brain
Spinal Cord
Peripheral Nervous System
cranial and spinal nerves)
Autonomic Nerves
Somatic Nerves
Nerves that carry signals to
and from smooth muscle, cardiac
muscle, and glands
Nerves that carry signals to
and from skeletal muscle,
tendons, and the skin
Sympathetic Parasympathetic
Division
Division
Two sets of nerves that often
signal the same effectors and
have opposing effects
Fig. 29.3, p. 469
Divisions of Vertebrate Nervous Systems
Central Nervous System
Brain
Spinal Cord
Peripheral Nervous System
(cranial and spinal nerves)
Autonomic Nerves
Somatic Nerves
Nerves that carry signals
to and from smooth muscle,
cardiac muscle, and glands
Nerves that carry signals
to and from skeletal muscle,
tendons, and the skin
Sympathetic Parasympathetic
Division
Division
Two sets of nerves that often
signal the same effectors and
have opposing effects
Stepped Art
Fig. 29.3, p. 469
The Human Nervous System
The Human
Nervous
cervical nerves
System
Brain
cranial nerves
(twelve pairs)
(eight pairs)
Spinal Cord
thoracic nerves
(twelve pairs)
ulnar nerve
(one in
each arm)
sciatic nerve
(one in each leg)
lumbar nerves
(five pairs)
sacral nerves
(five pairs)
coccygeal nerves
(one pair)
Fig. 29.4, p. 469
Key Concepts
• Nervous Systems
• Excitable cells called neurons form communication lines in
animal nervous systems
• Neurons of radially symmetrical animals connect as a
nerve net
• Neurons of bilaterally symmetrical animals are
concentrated in the head, and one or more nerve cords
run the length of the body
ANIMATION: Bilateral Nervous Systems
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Animation 13.12: Vertebrate nervous
system divisions
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ANIMATION: Comparison of Nervous
Systems
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29.3 Neurons: The Communicators
• The neuron’s nucleus and other organelles are in its cell body
• Cytoplasmic extensions (dendrites and axons) allow the
neuron to receive and send messages
• Electrical signals originate in the axon’s trigger zone, and
travel along the conducting zone to output zones (axon
terminals), which release signaling molecules
Key Terms
• dendrite
• Of a motor neuron or interneuron, a cytoplasmic extension
that receives chemical signals sent by other neurons and
converts them to electrical signals
• axon
• Neuron cytoplasmic extension that transmits electrical
signals along its length and secretes chemical signals at
its endings
A Motor Neuron
A Motor Neuron
dendrites
input zone
cell body
trigger zone
conducting zone
axon
output zone
axon terminals
Fig. 29.5, p. 470
ANIMATION: Functional zones of a motor
neuron
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Neuron Structure
• Sensory neurons typically have no dendrites; receptor
endings of the peripheral axon respond to a specific stimulus
• Interneurons have many signal-receiving dendrites and one
short axon
• Motor neurons have multiple, short dendrites, and a longer
axon with terminals that signal a muscle or gland cell
Neuron Structure
• Arrows indicate direction of information flow
Neuron Structure
receptor
endings
peripheral cell axon axon
axon
body
terminals
cell
body
axon
cell
body
axon
axon
terminals
dendrites
dendrites
A Sensory neuron with no
dendrites; receptor endings of
the peripheral axon respond to
a specific stimulus.
B Interneuron with short
dendrites and a short axon.
C Motor neuron with short
dendrites, and a longer axon
with terminals that signal a
muscle or gland cell.
Fig. 29.6, p. 470
ANIMATION: Neuron structure
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29.4 Membrane Potentials
• A neuron’s ability to send and receive messages stems from
electrical properties of its membrane
• The voltage difference across a neuron plasma membrane is
a membrane potential
• membrane potential
• Potential energy of charges separated by a cell membrane
Resting Potential
• Membrane potential across the axon of a neuron that is not
being excited (resting potential) is about -70 millivolts
• Resting potential arises from the action of transport proteins
in the neuron membrane, and the higher concentration of
negatively charged proteins inside the cell
• resting potential
• Membrane potential of a neuron at rest
Resting Potential
• The cytoplasm of a resting neuron has more negatively
charged proteins than interstitial fluid; transport protein move
K+ ions in and Na+ ions out of the neuron
Resting Potential
electrode
inside
electrode
outside
unstimulated axon
p. 471a
Action Potential
• Neurons and muscle cells are said to be “excitable” because
they can undergo an action potential – an abrupt reversal in
the electric gradient across the plasma membrane
• Charge reversal occurs when Na+ and K+ flow down their
concentration gradients through voltage-gated ion channels
• action potential
• Brief reversal of the charge difference across a neuron
membrane
Membrane Proteins in Neurons
• Sodium-potassium pumps actively transport 3 sodium ions
(Na+) out of a neuron for every 2 potassium ions (K+) they
pump in
• Passive transporters allow K+ ions to leak across the plasma
membrane, down their concentration gradient
• Voltage-sensitive channels for K+ or Na+ ions are shut when a
neuron is at rest, and open during an action potential
Membrane Proteins in Neurons
Membrane Proteins in Neurons
A Sodium–potassium
pumps actively
transport 3 sodium
ions (Na+) out of a
neuron for every 2
potassium ions (K+)
B Passive transporters
allow K+ ions to leak
across the plasma
membrane, down their
concentration gradient.
C Voltage-sensitive
channels for K+ or Na+
ions are shut when a
neuron is at rest (left).
They open during an
action potential (right).
Fig. 29.7, p. 471
Animation: Ion concentrations
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ANIMATION: Resting Potential
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Animation: Action Potential
29.5 A Closer Look at Action Potentials
• An action potential occurs only if a disturbance causes
membrane potential to rise to threshold potential
• threshold potential
• Neuron membrane potential at which voltage-gated
sodium channels open, causing an action potential to
occur
Approaching Threshold
• When a stimulus is intense or long-lasting, graded potentials
spread into a trigger zone
• The membrane in the trigger zone has many voltage-gated
sodium (Na+) channels
• When a stimulus shifts the voltage difference across the
membrane to a certain level (threshold potential) these
channels open and an action potential gets under way
An Action Potential (1)
• The trigger zone of a neuron contains sodium-potassium
pumps and voltage-gated ion channels
• When the membrane is at rest, the voltage-gated channels
are closed
• The cytoplasm’s charge is negative relative to interstitial fluid
An Action Potential (1)
An Action Potential (1)
interstitial fluid
with high Na+,
low K+
Na+/K+
pump
voltage-gated
ion channels
cytoplasm with
low Na+, high K+
Close-up of the trigger zone of a neuron. One sodium–potassium
pump and some of the voltage-gated ion channels are shown. At this
point, the membrane is at rest and the voltage-gated channels are
closed. The cytoplasm’s charge is negative relative to interstitial fluid.
1
Fig. 29.8.1, p. 472
An Action Potential (2)
• Arrival of a sufficiently large signal in the trigger zone raises
membrane potential to threshold level
• Gated sodium channels open: Na+ flows down its
concentration gradient into the cytoplasm
• Sodium inflow reverses the voltage across the membrane,
increasing inward flow of more ions (positive feedback)
• positive feedback
• A response intensifies the conditions that caused its
occurrence
An Action Potential (2)
An Action Potential (2)
Na+
Na+
Na+
Na+
Na+
Na+
Arrival of a sufficiently large signal in the trigger zone raises the
membrane potential to threshold level. Gated sodium channels open and
sodium (Na+) flows down its concentration gradient into the cytoplasm.
Sodium inflow reverses the voltage across the membrane.
2
Fig. 29.8.2, p. 472
An Action Potential (3)
• Charge reversal makes gated Na+ channels shut and gated
K+ channels open
• K+ outflow restores the voltage difference across the
membrane
• The action potential is propagated along the axon as positive
charges spreading from one region push the next region to
threshold
An Action Potential (3)
An Action Potential (3)
K+
K+
K+
Na+
Na+
Na+
The charge reversal makes gated Na+ channels shut and gated K+
channels open. The K+ outflow restores the voltage difference across the
membrane. The action potential is propagated along the axon as positive
charges spreading from one region push the next region to threshold.
3
Fig. 29.8.3, p. 473
An Action Potential (4)
• After an action potential, gated Na+ channels are briefly
inactivated, so the action potential moves one way only,
toward axon terminals
• Na+ and K+ gradients disrupted by action potentials are
restored by diffusion of ions that were placed by activity of
sodium-potassium pumps
An Action Potential (4)
An Action Potential (4)
Na+/K+
pump
K+
K+ K+
Na+
Na+
Na+
K+
After an action potential, gated Na+ channels are briefly inactivated, so
the action potential moves one way only, toward axon terminals. Na+ and
K+ gradients disrupted by action potentials are restored by diffusion of
ions that were put into place by activity of sodium–potassium pumps. Fig. 29.8.4, p. 473
4
ANIMATION: Action potential propagation
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An All-or-Nothing Spike
• Resting membrane potential is –70 mV
• Once threshold level (-60 mV) is reached, and an action
potential begins, membrane potential always rises to the
same peak level (+33 mV)
• An action potential is described as an all-or-nothing event
Changes in Membrane Potential
A Resting membrane
potential is –70 mV.
B Stimulation causes an
influx of positive ions and a
rise in the membrane
potential.
C Once potential exceeds
threshold (–60 mV), the
voltage-gated sodium (Na+)
gates begin to open, and Na+
rushes in. This causes more
gates to open, and so on.
Voltage shoots up rapidly.
Membrane potential (millivolts)
Changes in Membrane Potential
D
action potential
+30
C
threshold
level
-60
E
B
resting
level
-70
A
0
F
1
2
3
4
5
Time (milliseconds)
D Every action potential
peaks at +33 mV; no more,
no less. At this point, Na+
gates have closed and
potassium (K+) gates have
opened.
E Flow of K+ out of the
neuron causes the potential
to fall.
F So much K+ exits that
G potential declines below
resting potential.
G Action of the Na+/K+ pump
6 restores resting ion
concentrations.
Fig. 29.9, p. 473
ANIMATION: Measuring membrane
potential
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Propagation Along the Axon
• Action potentials travel in one direction: toward axon terminals
• Gated sodium channels cannot reopen right after they close
• Diffusion of sodium ions only opens gated sodium channels in
regions farther along the axon
ANIMATION: Action Potential
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